1. Field of the Invention
The present invention relates to a device and method for photodynamic therapy (PDT) that is capable of treating hyperproliferative tissue growth existing deeper in the body than is accessible to successful treatment by direct topical irradiation, while minimizing unnecessary surface skin damage.
2. Invention Disclosure Statement
There are various ways to treat hyperproliferative tissue growth in the body. Traditional approaches involve surgery, chemotherapy, and x-ray radiation to reduce or eliminate the tissue growth. A relatively new approach, photodynamic therapy (PDT), involves the use of photosensitizers and laser radiation provided through optical fibers to areas of tissue growth. PDT is a three step treatment process: in the first step, a photosensitive compound is administered systemically by injection or topically to a treatment site on the patient's body; and after a sufficient time to allow the photosensitizer to be absorbed, the treatment site is irradiated with a light having a wavelength corresponding to the characteristic absorption wavelength of the photosensitizer. The light activates the photosensitizer causing singlet oxygen radicals to be generated leading to biological effects that destroy the hyperproliferative tissue. The depth of penetration of the cytotoxic effect in the tissue depends on the depth of light penetration, the concentration and cellular distribution of photosensitizer in the tissue and the availability of molecular oxygen in the abnormal tissue or tumor.
PDT laser source wavelengths are selected based on the photosensitizer. It is seen that large tumors or tumors that are embedded beneath skin layers are difficult to reach and destroy completely because of the skin. Various devices and methods have been proposed to overcome this issue of deep lying tumors. Devices have been proposed that can treat these tissues to a limited depth and effectiveness of destruction. With a conventional external light source cytotoxicity in tissue during PDT is limited to a depth of about 5 mm, minimally for depth ranges from 5 mm to 10 mm and almost negligible for greater than 10 mm.
The outermost stratum corneum layer of the epidermis layer of the skin presents the most resistance to light irradiation. There is substantially less resistance to penetration in the subsequent epidermal and dermal layers of the skin. In order to target a large tumor or tumor residing deep in the skin layers, a means to deliver radiation to the target tissue is necessary for effective treatment.
Treatment of tumors by the use of an external light irradiation can lead to damage of healthy skin not only from the light but from the activation of the photosensitizer in that area. Photosensitizer accumulation is required to treat tumor tissue growth but with the availability of atmospheric oxygen in healthy skin layers followed by light activation or radiation can lead to unwanted skin damage. Various solutions have been proposed to eliminate these complications. In most topical radiation treatments, these upper layers of skin are damaged even when precautions are taken. Some methods use a cooling mechanism to minimize effects to the skin prior, during and after treatment.
U.S. Pat. No. 5,000,752 by Hoskin et al., entitled, “Treatment Apparatus and Method,” describes a method for transdermal laser delivery by the insertion of an array of needles fixed on a flexible plate into a port wine stain treatment site. Spear-like tips that aid in the breaking of the skin surface and diffusing of the radiation are attached to the needles; optical fibers are attached to the tips within the needles. The tips have a diameter equal to the diameter of the needles and may cause unnecessary surface skin damage. The needles have specialized and adjustable tips which makes the device expensive and difficult to sterilize. Since the tip of the needle must be in the area of treatment, a larger diameter hole is created into the skin. Further, adjustment of the depth of the needle is not disclosed and therefore treatment of hyperproliferative tissue beyond the port wine stain deep is not achieved as in the present invention with minimum of damage to the tissue.
In Publication US 2004/0215292 by Chen, a method is defined for transcutaneously administering photodynamic therapy to a target tissue that lies deep below the skin. The method involves the introduction of a photosensitizing agent or a photosensitizing agent delivery system or a prodrug product that selectively binds to the target tissue. A low intensity light with a waveband corresponding to the absorption wavelength of the photosensitizer is applied for a long duration of time. After the drug binds to the target tissue and upon initiation of light activation in the surrounding tissue there results blood vessel closure/thrombosis in the surroundings. This is followed by targeting the treatment site with irradiation to ensure damage of the tumor. This is a very lengthy process of irradiating target tissue with a low fluence of radiation for about 3 to 24 hours to reduce skin or surrounding tissue damage.
An ideal device for the present invention would be one able to protect the stratum corneum (˜20 μm), the epidermis (˜100 μm) and the dermal (˜1200 μm) layers, and deliver radiation beyond the hypodermal (˜1200 μm) layer of skin where the healthy surface skin layers are minimally irradiated. The device would be one which will deliver light irradiation to the deep lying tumor in order to provide uniform tissue destruction at the treatment site. Hence a device and method is needed to target the deep and large hyperproliferative tissue with improved laser delivery without the complications associated with the prior art.